1. Field of the Invention
The invention relates to a method and a device for decoding convolution-encoded signals, in particular, reception signals in mobile telephony and radio telephony.
2. Discussion of the Background
A decoder based upon the Viterbi algorithm is conventionally used for decoding convolution-encoded signals. In the case of convolution-encoded signals, the data to be transmitted correspond to the states of a “hidden Markov” model. The actually-transmitted data correspond to the state transitions in the model. From the visible state transitions, inferences are drawn through the Viterbi algorithm regarding the states at every timing point. For this purpose, a metric is determined, which, with a given reception symbol, indicates the probability of every state transition from every possible present state to every possible following state. Accordingly, the Euclidean distance of the present reception symbol to the symbols of the state transitions is determined. In this context, each individual state transition is described as a branch or a bough. A path is formed by connecting the individual branches in a sequence. The metric of the path is obtained by summation of the individual branch metrics.
The optimum path and therefore the most probable sequence of transmission symbols are given by the path of minimum metric. In the case of convolution-encoded signals, every reception symbol contains components of a plurality of symbols arranged in time succession. This is referred to as the memory of the code. Accordingly, the influence of a past reception symbol on the present state is limited. After a given number of reception symbols, any influence of the past reception symbol on the present state is no longer determinable. After the passage of this given number of reception symbols, the number of the paths, which enter the further calculation, is reduced to the most probable path. This step is referred to as the elimination of improbable paths. With this procedure, an optimum receiver for the convolution-encoded signal is provided.
For example, a device for synchronisation in a Viterbi decoder and therefore also its function are shown in U.S. Pat. No. 3,872,432 B1.
However, one disadvantage of the Viterbi algorithm is the marked dependence of the processing cost upon the parameters of the convolution-encoded signal. Accordingly, the cost rises very steeply with a large number of modulation stages and also with a long code memory.
Alternatively, convolution-encoded signals are decoded with the use of “minimum mean-square-error decision-feedback equaliser” (MMSE-DFE) decoders. Directly upon receipt of the symbol, these decoders finally determine the present state from the past reception symbols, without taking future reception symbols into consideration.
An MMSE-DFE decoder is shown in DE 199 48 373 A1.
These decoders in fact resolve the problem of the high processing cost of the Viterbi decoder, however, they provide a substantially reduced detection security. For example, especially in the case of strongly-disturbed transmission channels, a high bit-error rate of the reception signal is achieved.